280 CLARKE— THE AGE OF THE EARTH. 



have indicated for so many different groups of life, and can compel 

 our conception to grasp the duration of the vast unrecorded past of 

 life-history, there remains another phase of the paleontological record 

 which in part emphasizes and in part serves as a check on this con- 

 ception. It has fallen to me to study the earliest recorded expressions 

 of dependent life — that is, the beginnings, so far as we can find them, 

 of such consociations of animals as we are wont to designate as para- 

 sitic, mutualistic, and symbiotic, wherein one creation has depended 

 upon or adjusted itself to the life functions or habits of another, or 

 has sought mechanical protection at the cost of its own locomotive 

 independence. Two very obvious facts seem to stand out as a result 

 of these inquiries : ( i ) That these interdependent conditions with 

 which the living world is rife to-day, in passing backward to the early 

 stages of Paleozoic time, become palpably fewer; indeed, while such 

 conditions are well marked in some groups and common in others 

 during the middle and later Paleozoic, they are very unusual in the 

 earlier stages and in the Cambrian fauna are little more than sug- 

 gested. (2) This dependent state seems with reasonable clarity to 

 be resolvable into an original loss of locomotive independence, a will- 

 ingness to be fed rather than to feed, an adaptation to an easier mode 

 of life. The commanding percentage of the Cambrian fauna belongs 

 to groups against which the charge of surrender of locomotive inde- 

 pendence can hardly be laid, though inclusive of groups of animals 

 which in later stages did become infected with the loss of independ- 

 ence, but still in a capital sense embraced those whose independent 

 living was unimpaired. 



These considerations I have analyzed elsewhere in some detail 

 and their significance is this — that the degeneration of life (for de- 

 pendence of necessity implies degeneration of physiology) has been 

 a process attendant upon and of course influencing evolution, but 

 apparently limited in its effects to that part of the procession of life 

 which comes under our actual observation ; that is, since the days of 

 the free and independent faunas of the Cambrian. If this is an 

 approximation to the truth, as we believe it to be, then in a broad 

 sense the real vigor of life, which established the major branches and 

 laid down the plan of all future ages, was dominant in its purity in 



